1. Field of the Invention
The present invention relates to mutations of the subtilisin gene which result in changes in the chemical characteristics of subtilisin enzyme. Mutations at specific nucleic acids of the subtilisin gene result in amino acid substitutions and consequently, altered enzyme function. Some of these mutant enzymes exhibit physical properties advantageous to industrial applications, particularly in the detergent industry, providing subtilisin which is more stable to oxidation, possesses greater protease activity, and exhibits improved washability.
2. Description of Related Art
Bacillus Proteases
Enzymes cleaving the amide linkages in protein substrates are classified as proteases, or (interchangeably) peptidases (See Walsh, 1979, Enzymatic Reaction Mechanisms. W.H. Freeman and Company, San Francisco, Chapter 3). Bacteria of the Bacillus species secrete two extracellular species of protease, a neutral, or metalloprotease, and an alkaline protease which is functionally a serine endopeptidase, referred to as subtilisin. Secretion of these proteases has been linked to the bacterial growth cycle, with greatest expression of protease during the stationary phase, when sporulation also occurs. Joliffe et al. (1980, J. Bacterial. 141:1199-1208) has suggested that Bacillus proteases function in cell wall turnover.
Subtilisins
A serine protease is an enzyme which catalyses the hydrolysis of peptide bonds, in which there is an essential serine residue at the active site (White, Handler, and Smith, 1973, “Principles of Biochemistry,” Fifth Edition, McGraw-Hill Book Company, NY, pp. 271-272).
The serine proteases have molecular weights in the 25,000 to 30,000 range. They are inhibited by diisopropylfluorophosphate, but in contrast to metalloproteases, are resistant to ethylenediamine-tetra acetic acid (EDTA) (although they are stabilized at high temperatures by calcium ion). They hydrolyze simple terminal esters and are similar in activity to eukaryotic chymotrypsin, also a serine protease. The alternative term, alkaline protease, reflects the high pH optimum of the serine proteases, from pH 9.0 to 11.0 (for review, see Priest, 1977, Bacteriological Rev. 41:711-753).
A subtilisin is a serine protease produced by Gram-positive bacteria or fungi. A wide variety of subtilisins have been identified, and the amino acid sequences of at least eight subtilisins have been determined. These include six subtilisins from Bacillus strains, namely, subtilisin 168, subtilisin BPN′, subtilisin Carlsberg, subtilisin DY, subtilisin amylosacchariticus, and mesentericopeptidase (Kurihara et al., 1972, J. Biol. Chem. 247:5629-5631; Stahl and Ferrari, 1984, J. Bacteriol. 158:411-418; Vasantha et al., 1984, J. Bacteriol. 159:811-819, Jacobs et al., 1985, Nucl. Acids Res. 13:8913-8926; Nedkov et al., 1985, Biol. Chem. Hoppe-Seyler 366:421-430; Svendsen et al., 1986, FEBS Lett 196:228-232), and two fungal subtilisins, subtilisin thermitase from Thermoactinymyces vulgaris (Meloun et al., 1985, FEBS Lett. 183:195-200) and proteinase K from Tritirachium album (Jany and Mayer, 1985, Biol. Chem. Hoppe-Seyler 366:584-492).
Subtilisins are well-characterized physically and chemically. In addition to knowledge of the primary structure (amino acid sequence) of these enzymes, over 50 high resolution X-ray structures of subtilisin have been determined which delineate the binding of substrate, transition state, products, three different protease inhibitors, and define the structural consequences for natural variation (Kraut, 1971, Ann. Rev. Biochem. 46:331-358). Random and site-directed mutations of the subtilisin gene have both arisen from knowledge of the physical and chemical properties of the enzyme and contributed information relating to subtilisin's catalytic activity, substrate specificity, tertiary structure, etc. (Wells et al., 1987, Proc. Natl. Acad. Sci. U.S.A. 84:1219-1223; Wells et al., 1986, Phil. Trans. R. Soc. Lond. A. 317:415-423; Hwang and Warshel, 1987, Biochem. 26:2669-2673; Rao et al., 1987, Nature 328:551-554).
Industrial Applications of Subtilisins
Subtilisins have found much utility in industry, particularly detergent formulations, as they are useful for removing proteinaceous stains. To be effective, however, these enzymes must not only possess activity under washing conditions, but must also be compatible with other detergent components during storage. For example, subtilisin may be used in combination with amylases, which are active against starches; cellulases which will digest cellulosic materials; lipases, which are active against fats; peptidases, which are active on peptides, and ureases, which are effective against urine stains. Not only must the formulation protect other enzymes from digestion by subtilisin, but subtilisin must be stable with respect to the oxidizing power, calcium binding properties, detergency and high pH of nonenzymatic detergent components. The ability of the enzyme to remain stable in their presence is often referred to as its washing ability or washability.